Summary of Work: Somatic and germinal mutations can have severe impact on the fitness of multicellular organisms and offspring. The ability to study the occurrence of mutations in higher organisms has been limited by available methods rendering it difficult to address important questions such as 1) kinetics of mutation induction in relation to organ specificity, 2) mutation induction during development, 3) systemic effects of somatic mutations in age-related degenerative diseases, and 4) tumor progression from pre-neoplastic to neoplastic growth in relation to genomic stability. Initially, PhiX174 with the well characterized am3 mutation is used as a transgene to evaluate substitutions at the A:T base pair. The animals for mutagenesis studies were produced by mating am54 males to C57BL6/J females. A procedure was developed to measure the mutations that are fixed in the animal independently from the mutations that arise from in vivo and ex vivo damage in the DNA. Hemizygous male offspring from this cross were injected i.p. with various doses of ENU. The revertant frequency increased linearly with the ENU dose only among the revertants that were fixed in the mouse and not among the revertants that were due to DNA damage. The PhiX system is presently the only system where a clear distinction can be made between mutations that are fixed in the animal and mutations that arise from DNA damage present in the isolated DNA